With the development and progress of electronic technology, power electronic systems tend to become integrated and digital. Series-parallel converter system is system-level integration and, by means of series and parallel of standardized modules, configured to achieve functions for different applications. At present, the technical difficulty of series-parallel converter system lies in voltage and current sharing, and it is relatively difficult to deal with current sharing while there is precision requirement for input/output voltage sharing. Accordingly, an outer current sharing loop or a conventional droop characteristic current sharing control is commonly adopted.
When the outer current sharing loop is adopted, an additional control loop is needed. The bandwidth of the control loop is affected greatly by the communication delay. When the power conversion is digitized, the communication delay will affect the control performance of the converter (including dynamic performance, system response speed, etc.). Thus, the current share is relatively slow with poor effect of dynamic current sharing and is susceptible to interference of ambient noise. When the system is relatively complex and there is multiple loop control, the part of current sharing loop is to be kept as simply as possible. Additional design of the current sharing loop, however, may increase the complexity of the system. On the other hand, in the case of hot-swap, the lack of converter data acquisition may cause disorder to update of communication control parameter.
Conventional droop characteristic current sharing control can realize wireless autonomous current sharing, so as to avoid the problem of wired current sharing, but the introduction of control deviation on the input voltage may lead to poor efficiency of voltage regulation and poor precision of voltage sharing.
As the conventional droop characteristic current sharing control is adopted, FIG. 1 illustrates a droop characteristic curve corresponding to the input voltage control of two parallel converters. A virtual impedance coefficient R is indicative of a drooping speed of the input voltage VINi of the i-th parallel converter with the increase of the output current IO. Herein, the input voltage is as shown in equation (1) below:
                              V                      IN            ⁢                                                  ⁢            i                          =                                                            -                R                            ·              I                        ⁢                                                  ⁢            o                    +                                    1              2                        ⁢                          V              busi                                                          (        1        )            
wherein Vbus is the total input voltage of the system and
      1    2    ⁢      V    busi  is the reference voltage of the i-th parallel converter.
As shown in FIG. 1 and equation (1), after introduction of the virtual impedance coefficient R in the i-th parallel converter, a control deviation ΔVINi of the input voltage is generated at the input voltage VINi of the i-th parallel converter with the increase of the output current IO of the i-th parallel converter (where i=1, 2 L, n, nϵN), as shown in the following equation (2):
                              Δ          ⁢                                          ⁢                      V            INi                          =                                                            1                2                            ⁢                              V                busi                                      -                          V              INi                                =                      Io            ·            R                                              (        2        )            
It can be seen that the control deviation of the input voltage is generated in the input voltage of the i-th parallel converter due to the virtual impedance coefficient R, and the control deviation increases as the output current IO of the i-th parallel converter increases. The voltage sharing of the input voltage is thus affected in the series-parallel converter system, and the greater the R, the poorer the sharing effect.
On the other hand, as shown in FIG. 1, when the droop control is performed, ΔVerror between two parallel converters is constant, while the unbalance current ΔI affecting the current sharing effect becomes smaller as the R increases, as shown in the following equation (3).ΔI=ΔVerror·cot θ=ΔVerror/R  (3)
wherein ΔI is the unbalance current of output currents corresponding to the two parallel converters, ΔVerror is the control deviation of input voltages between the two parallel converters, and θ is an angle between the drooping curve and the horizontal line (supplementary angle corresponding to a slope angle of the drooping curve).
Therefore, as can be seen from the equation (3), a magnitude of the virtual impedance R is associated with that of the unbalanced current ΔI. Increasing the value of R can significantly reduce the unbalanced current, that is, enhance the current sharing effect.
In summary, for a series-parallel system in which the virtual impedance R is introduced, there is a contradiction between the voltage and current sharing control. It is a technical obstacle in the combination of series-parallel converters to improve the control deviation of input voltage while improving accuracy of current sharing through the conventional droop characteristic. So far, there are many shortcomings in the existing technical methods for handling this obstacle.
Therefore, it is an urgent problem to be solved at present to provide a technical solution which can further improve the above-mentioned drawbacks.